Flat tube heat exchangers are employed in a wide variety of situations for transferring heat between a first fluid flow and a second fluid flow. One specific example of where flat tube heat exchangers can be employed is in fuel cell systems for transferring heat between fluid flows to improve overall system efficiency. Specifically, heat exchangers may be utilized in fuel processing subsystems of fuel cell systems to transfer heat between a reformate flow and a coolant flow to cool the reformate flow prior to entry into a carbon monoxide removal unit.
In many PEM fuel cell systems, a fuel such as methane or a similar hydrocarbon fuel is converted into a hydrogen-rich stream for the anode side of the fuel cell. In many systems, humidified natural gas (methane) and air are chemically converted to a hydrogen-rich stream known as reformate by a fuel processing subsystem of the fuel cell system. This conversion takes place in a reformer where the hydrogen is catalytically released from the hydrocarbon fuel. A common type of reformer is an Auto-Thermal Reactor (ATR), which uses air and steam as oxidizing reactants. As the hydrogen is liberated, a substantial amount of carbon monoxide (CO) is created which must be reduced to a low level (typically less than 10 ppm) to prevent poisoning of the PEM membrane.
The catalytic reforming process consists of an oxygenolysis reaction with an associated water-gas shift [CH4+H2O→CO+3H2, CO+H2O→CO2+H2] and/or a partial oxidation reaction [CH4+½ O2→CO+2H2]. While the water-gas shift reaction removes some of the CO from the reformate flow stream, the overall reformate stream will always contain some level of CO, the amount being dependent upon the temperature at which the reforming process occurs. After the initial reactions, the CO level of the reformate flow is well above the acceptable level for the PEM fuel cell. To reduce the CO concentration to within acceptable levels, several catalytic reactions will generally be used in the fuel processing subsystem to remove CO in the reformate flow. Typical reactions for reduction of CO in the reformate flow include the aforementioned water-gas shift, as well as a selective oxidation reaction over a precious metal catalyst (with a small amount of air added to the reformate stream to provide oxygen). Generally, several stages of CO cleanup are required to obtain a reformate stream with an acceptable CO level. Each of the stages of CO cleanup requires the reformate temperature be reduced to precise temperature ranges so that the desired catalytic reactions will occur and the loading amount of precious metal catalyst can be minimized.
In this regard, liquid-cooled heat exchangers are frequently employed to control the reformate temperature at each stage because of their compact size when compared to gas-cooled heat exchangers. Because liquid water entering the fuel processing subsystem must be heated so that it can be converted to steam for the reforming reactions, it is thermally efficient to use process water as the liquid coolant for the heat exchangers to cool the reformate flow prior to CO removal.
However, utilizing liquid-cooled heat exchangers to cool the reformate flow prior to entering CO removal units have a few conditions that should be considered. For example, the temperature of the reformate flow exiting the heat exchanger needs to be relatively precisely controlled so the CO removal processes can be optimized. Another factor to consider is the difference in flow rates between the reformate flow and the coolant flow.